Mass spectrometry (MS) plays a major role in chemical, biological and geological research. Proteomic, glycomic, lipidomic and metabolomic studies would be impossible without modern mass spectrometry. Owing to their high sensitivity and exceptional specificity, mass spectrometric methods also appear to be ideal tools for in vivo analysis in the life sciences. In many of these applications, however, the samples must be preserved in their native environment with preferably no or minimal interference from the analysis. For most of the traditional ion sources applied in the biomedical field, such as matrix-assisted laser desorption ionization (MALDI) or electrospray ionization (ESI), these limitations present serious obstacles. For example, MALDI with ultraviolet laser excitation requires the introduction of an external, often denaturing, matrix, whereas ESI calls for liquid samples with moderate ionic conductivity. As living organisms are typically disrupted by such preparations, there is a great interest in developing direct sampling and ambient ionization sources for in vivo studies.
Rapid advances in recent years have provided a growing number of ambient ion sources. For example, atmospheric pressure infrared MALDI (AP IR-MALDI), capable of producing ions from small and moderate size molecules (up to 3,000 Da), shows promise for metabolic imaging. Small molecules have been analyzed by other methods, including direct analysis in real time (DART), desorption electrospray ionization (DESI), desorption atmospheric pressure chemical ionization (DAPCI) and matrix-assisted laser desorption electrospray ionization (MALDESI). Medium to large biomolecules have also been detected by DESI and on dehydrated samples by electrospray laser desorption ionization (ELDI). Imaging capabilities were demonstrated for DESI on a rat brain tissue section with about 400 μm lateral resolution. Due to the need for sample pretreatment, sensitivity to surface properties (DESI, DART, DAPCI and AP IR-MALDI) and external matrix (ELDI and MALDESI), in vivo capabilities are very limited for these techniques.
An awkward feature of mass spectrometry (MS) is the requirement of a vacuum system. Analysis under ambient conditions would simplify and expand the utility of mass spectrometry.
Takats et al. report a method of desorption electrospray ionization (DESI) whereby an aqueous spray of electrosprayed charged droplets and ions of solvent are directed at an analyte which has been deposited on an insulating surface. The microdroplets from the aqueous spray produce ions from the surface whereby the desorbed ions are directed into a mass spectrometer for analysis. A broad spectrum of analytes was examined, including amino acids, drugs, peptides, proteins, and chemical warfare agents.
Cody et al. report a method they called “DART” wherein helium or nitrogen gas is sent through a multi-chambered tube wherein the gas is i) subjected to an electrical potential, ii) ions are removed from the gas stream, iii) the gas flow is heated, and then iv) the gas is directed at a mass spectrometer ion collection opening. They report that subjecting hundreds of different chemicals to this technique provided a very sensitive method for detecting chemicals, including chemical warfare agents and their signatures, pharmaceuticals, metabolites, peptides, oligosaccharides, synthetic organics and organometallics, drugs, explosives, and toxic chemicals. Further, they report that these chemicals were detected on a wide variety of substrates including concrete, asphalt, skin, currency, airline boarding passes, business cards, fruit, vegetables, spices, beverages, bodily fluids, plastics, plant leaves, glassware, and clothing.
Shiea et al. report the development of a method called electrospray-assisted laser desorption ionization (ELDI). They report that DESI-MS is limited in that it cannot analyze complex mixtures and there is very little control over the size and definition of the surface area affected by the ESI plume for the desorption of the analyte. They also acknowledge the problem that direct laser desorption is limited to low molecular weight compounds and that lasers desorb more neutrals than ions. Accordingly, they report a combination of ESI and ultraviolet laser desorption (LD) wherein i) a sample is irradiated with a pulsed nitrogen laser beam to generate laser desorbed material, ii) this material is then ionized by subjecting it to an electrospray plume, and iii) the ions sent to a mass spectrometer. This technique is reported to provide sensitivity towards protein detection without sample prep or the use of a matrix. However, their experimental setup shows a stainless steel sample plate upon which aqueous solution of protein was spread and the sample dried. The method was ultimately presented for the analysis of solid samples.
Atmospheric pressure laser desorption techniques such as atmospheric pressure matrix-assisted laser desorption ionization (AP-MALDI) or electrospray-assisted laser desorption ionization (ELDI) usually require the pretreatment of the sample with a suitable matrix.
Further, it has been difficult previously to study the spatial distribution of chemicals at atmospheric pressure using MS.
Lastly, other matrixless methods do not achieve ESI-like ionization. Thus, with other matrixless methods (e.g., DIOS) large molecules cannot be detected as multiply charged species.
The following documents may provide additional context where necessary for fuller understanding of the claimed invention and are incorporated by reference herein in their entirety for references purposes and for determining the level of ordinary skill in the art:
U.S. Pat. Nos. 6,949,741 and 7,112,785 by Cody et al.
U.S. Pat. No. 5,965,884 by Laiko et al.;
Publication on DESI: “Mass Spectrometry Sampling Under Ambient Conditions with Desorption Electrospray Ionization,” Z. Takats; J. M. Wiseman; B. Gologan; and R. G.
Cooks, Science 2004, 306, 471-473;
Publication on ELDI: “Direct Protein Detection from Biological Media through Electrospray-Assisted Laser Desorption Ionization/Mass Spectrometry,” M.-Z.
Huang; H.-J. Hsu; J.-Y. Lee; J. Jeng; J. Shiea, J. Proteome Res. 2006, 5, 1107-1116; and Publication on DART: “Versatile New Ion Source for the Analysis of Materials in Open Air under Ambient Conditions,” R. B. Cody; J. A. Laramee; and D. Durst, Anal. Chem. 2005, 77, 2297-2302.